Distortion Problem with USB from PC to DAC

Strange distortion problem with USB connect from my PC to my Cambridge DacMagic Plus. When I first attach my USB cable to the DAC, everything sounds great, then after a couple of minutes I get a buzzing sort of distortion in both channels that gradually increases to the point where I stop playback and unplug the USB because I think something could possibly be damaged. I have tried different cables (all cheap), I even tried different PC's, one of them a Laptop thats not connected to wall power, same problem. One thing I noticed is that when I unplug the USB from the DAC, the connector is very warm! I notice the problem mostly listening to my headphone amp. Any ideas? Thanks!

grm